Stem cells are primal cells common to all multi-cellular organisms that retain the ability to renew themselves through cell division and can differentiate into a wide range of specialised cell types. The two broad categories of mammalian stem cells exist: embryonic stem cells, derived from blastocysts, and adult stem cells, which are found in adult tissues. In a developing embryo, stem cells are able to differentiate into all of the specialised embryonic tissues. In adult organisms, stem cells and progenitor cells act as a repair system for the body, replenishing specialised cells. As stem cells can be readily grown and transformed into specialised tissues such as muscles or nerves through cell culture, their use in medical therapies has been proposed. In particular, embryonic cell lines, autologous embryonic stem cells generated therapeutic cloning, and highly plastic adult stem cells from the umbilical cord blood or bone marrow are touted as promising candidates. Among the many applications of stem cell research are nervous system diseases, diabetes, heart disease, auto-immune diseases as well as Parkinsons disease, end-stage kidney disease, liver failure, cancer, spinal cord injury, multiple sclerosis, Parkinsons disease, and Alzheimers disease. Stem cells are self-renewing, unspecialised cells that can give rise to multiple types all of specialised cells of the body. Stem cell research also involves complex ethical and legal considerations since they involve adult, foetal tissue and embryonic sources. This book presents important research in this field.